How you answer this question will determine how well you live in retirement

Every decision you make with regard to your retirement plan hangs on one question: How long am I going to live?

Accurately estimating your life span requires you to be honest with yourself about your family history and health habits.

At the same time, there are financial moves you may want to consider so that you do not run out of money in your old age.

Sam Edwards | Getty Images

No one knows how long they will live. Yet how realistic your estimate is can make or break your retirement plans.

When you decide about claiming Social Security retirement benefits, one of the first questions you need to ask is how long you may live. And when you decide how much you will save toward retirement, your longevity is a key factor you need to consider.

But the problem is that a lot of people fail to hit the mark.

"Whenever we do our surveys, we always ask people, 'How long do you expect to live?'" said Jamie Hopkins, professor of retirement planning at The American College. "What we always see is people underestimate how long they're going to live."

In 2016, life expectancy at birth in the U.S. was 78.6 years, according to the Centers for Disease Control and Prevention. For men, it was 76.1 years, while for women it was 81.1.

The leading causes of death are heart disease, cancer and accidents, according to the agency.

But once you live to a certain age — say 65 — that increases the likelihood that you will live into your old age.

But if you have not planned to live that long, you could end up broke.

"What we don't want to have happen is wake up on our 95th birthday or 100th birthday and find out that we're out of money," said Catherine Collinson, president and CEO of the Transamerica Institute and Transamerica Center for Retirement Studies.

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Make sure your calculation is realistic

No one has a crystal ball to predict how long they are going to live, Collinson said. But you can take steps to make sure your projected life span is accurate.

Using an online calculator or working with a financial advisor to come up with an estimate is a great place to start. But keep in mind that that number is just an approximation, and those calculations can have flaws.

Collinson's own financial advisor pegged her life expectancy at 95. But her grandmother lived to be 97.

As a result, Collinson decided to plan for a longer life than the calculator projected.

Keep in mind that average life expectancies are just that — an average. That means that some individuals will not live to that age while others will pass it, Hopkins said.

"Mostly we're going to modify it up," Hopkins said. "If we're doing planning, the risk is not so much dying younger.

"It's outliving our assets in retirement."

Be real about your personal situation

It may be tempting to answer certain questions less honestly — like how much you really drink, smoke or exercise per week.

But keep in mind that those little white lies can greatly offset your estimated life span and your financial plan.

Family health history is one metric that is used. But so is your own health status and your ability to change it.

Medical care and advances could significantly extend your life.

"There's tremendous potential that people can live even longer than what is in the life expectancy tables," Collinson said. "We should be planning for that."

Carolyn McClanahan, founder and director of financial planning at Life Planning Partners, said she had to convince one client who had had a minor stroke and a heart attack that he would live past 72, the age at which he decided he was going to prematurely die.

That client wanted to give his money away to his children. But McClanahan convinced him not to. The reason: He was getting quality health care and taking all the steps he needed to, and his life expectancy was actually 84.

Today, that client is 75.

The lesson: "Don't go blowing your financial plan," McClanahan said.

Stretch your income

There are a couple of steps you can take to make sure you do not outlive your money.

By delaying Social Security retirement benefits until the maximum age — 70 years old — you can ensure you get the highest monthly check available to you.

If you cannot wait until then, your full retirement age — 66 or 67 years old for most — is the point at which you will receive your full benefit based on your work record.